Electricity: measuring and testing – Electromechanical switching device – Relay
Reexamination Certificate
1999-02-08
2001-03-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electromechanical switching device
Relay
C219S109000
Reexamination Certificate
active
06208146
ABSTRACT:
TECHNICAL FIELD
This invention relates to resistance spot welding simulations and more particularly to a method and apparatus for measuring contact resistance for spot welding simulations.
BACKGROUND OF THE INVENTION
The resistance spot welding process is commonly used to make spot welds to join sheet metal parts. For example, thousands of spot welds are used in assembling the automobile. Therefore, improving the resistance spot welding characteristics has direct impact on the cost as well as the quality of automotive productions. Electrical contact resistance at electrode-to-sheet metal and sheet-to-sheet metal interfaces is an important characteristic to determine in order to optimize the welding process. There are currently two methods used to obtain electrical contact resistance measurements. One method is called an on-line approach, and the other one is called an off-line approach. The on-line approach indirectly monitors the variation of contact resistance during the real spot welding process. A problem with the on-line approach is that because there is an increase in heat locally, it is difficult to obtain the contact resistance property at a controlled temperature.
The off-line approach measures the contact resistance by simulating conditions during the real process, such as temperature and compressive load. A small current, as typically used in multimeter measurements, is used to measure the resistance in the off-line approach. A problem with the off-line approach is that, because of the existence of surface oxide or contaminant film, the measurements are strongly dependent upon the surface conditions and thus measurements are often inaccurate.
SUMMARY OF THE INVENTION
The present invention is directed to a method and apparatus for accurately measuring the contact resistance between electrode-to-workpiece and workpiece-to-workpiece interfaces by eliminating the problems associated with varying temperatures and contaminant films on surfaces of workpieces to be welded. The method of the invention includes applying a constant compressive load through electrodes and workpieces to simulate a load applied during an actual resistance spot welding process. The electrodes and workpieces are maintained at a constant temperature. A high current of, e.g., 1.2 KA is applied to the workpieces to break down contaminant films on surfaces of the workpieces. By breaking down the films, a more accurate reproducible measurement of contact resistance is obtained. Then a small current of, e.g., 2A is applied to the electrodes and workpieces and the voltage drop across and the current flow through the electrodes and workpieces is measured. From the voltage and current measurements, the contact resistance is determined.
The apparatus of the present invention includes a heating system, cooling system, loading system, discharging current circuit and resistance measuring system. The apparatus is capable of measuring contact resistance under different load and temperature conditions. The heating system maintains the desired temperature conditions for the measurements. The cooling system prevents the heater from overheating and cools the electrodes. A compressive load is applied to the electrodes and workpieces in order to simulate the load applied during resistance spot welding. The discharging current circuit is connected across the electrodes and applies an initial high current (suitably 1.2 KA) through the electrodes and workpieces to eliminate the contaminant film on the surfaces of the workpieces. Resistance measurement is achieved by applying a small current (suitably 2A) through the electrodes and workpieces and then measuring the voltage drop across the electrodes and workpieces. Then, the contact resistance can be determined from the measured voltage across and the current through the electrodes and workpieces.
These and other features and advantages of the invention will be more fully understood from the following description of a certain specific embodiment of the invention taken together with the accompanying drawings.
REFERENCES:
patent: 3654424 (1972-04-01), Vanderhelst
patent: 4497994 (1985-02-01), Flemm et al.
patent: 5021625 (1991-06-01), Destefan et al.
patent: 5260668 (1993-11-01), Mallory et al.
patent: 5977505 (1999-11-01), Kobayyashi et al.
Huang Ningjian
Wang Pei-Chung
General Motors Corporation
Grove George A.
Metjahic Safet
Nguyen Vincent Q.
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